1. Field of the Invention
This invention generally relates to a ghost cancelling device and more particularly to a ghost canceller for use in various kinds of television (TV) equipment and video equipment which perform processing of TV broadcasting waves and video signals obtained by detecting the TV broadcasting waves in order to remove ghost or waveform distortion included in input video signals.
2. Description of the Related Art
In recent years, there has been a trend of development of a display for use in a TV receiver and so on toward a high definition and/or large-screen display. Reflecting this trend, a high definition TV broadcasting system of which typical examples are a "High Vision" and "Clear Vision" Systems has been attracting attention. On the other hand, a ghost (i.e., a multiple image) interference is generated by simultaneously receiving direct waves and indirect waves reflected by, for example, a towering building and a hillside presents a serious problem again. Moreover, a recent increase in towering buildings and so forth has given rise to a rapid expansion of areas in which the ghost interference occurs. Thus, in such areas, there is necessity of reducing and further cancelling ghosts in a TV receiver. Especially, a high-grade TV receiver is demanded to be provided with a ghost canceller for elimination of ghosts from input video signals.
To meet such a demand, a reference signal for cancelling a ghost is inserted in NTSC (National Television System Committee) TV video signals in Japan. More particularly, the reference signal for cancelling a ghost is inserted into a predetermined line of a vertical retrace line interval. A reference pulse is generated from the reference signal for cancelling a ghost and is then used to perform a ghost cancelling operation. The details of such ghost cancelling operation are explained in many documents such a Japanese Magazine "Nikkei Electronics", 1989 8.7 (No. 479), page 121, and Japanese Magazine "Houso Gijyutsu", April. 1989.
Hereinafter, a method of generating the reference pulse will be described with reference to waveform diagrams of FIG. 3.
As illustrated in FIGS. 3(A)-(J), a reference signal for cancelling a ghost comprises four pairs of a GCR (Ghost Cancel Reference) and pedestal signals, each of which corresponds to a field, inserted in the predetermined line as four phases. Further, a period of repetition of the four phases is eight fields. Incidentally, a GCR signal (see FIGS. 3(A), (C) and so on) is a reference signal (hereunder referred to as a detection reference signal) for detecting a ghost, and a pedestal signal (see FIGS. 3(B), (D) and so on) pairing the GCR signal is an auxiliary signal (hereunder sometimes referred to also as a detection reference signal) for extracting a waveform of a bar-like signal employed as the GCR signal by effecting a subtraction between signals which are distant in time from each other by four fields to cancel a horizontal synchronization signal and a burst signal. The GCR signals are inserted into a first, third, sixth and eighth fields (respectively corresponding to FIGS. 3(A), (C), (F) and (H)). In contrast, the pedestal signals are inserted into a second, fourth, fifth and seventh fields (respectively corresponding to FIGS. 3(B), (D), (E) and (G)).
Incidentally, in this figure, superscripts "+" and "-" written above and to the right side of reference characters "GCR" indicating a GCR signal and of reference numeral "0" indicating a pedestal signal represent polarities of a burst signal corresponding to the predetermined line. As can be understood from FIG. 3, the horizontal synchronization signal and the burst signal are cancelled by a subtraction between signals which are distant in time from each other by four fields, so that positive and negative waveforms of bar-like signals employed as GCR signals respectively illustrated in FIGS. 3(I) and (J) can be extracted. Incidentally, in FIGS. 3(I) and (J), reference characters +GCR and -GCR represent a GCR signal having the positive waveform of a bar-like signal (hereunder referred to as a positive bar-like GCR signal) and another GCR signal having the negative waveform of a bar-like signal (hereunder referred to as a negative bar-like GCR signal), respectively. To obtain a pulse-like reference signal (hereunder sometimes referred to simply as a reference signal), it is necessary to change the GCR signal (i.e., the bar-like signal) into a pulse-like signal as illustrated in FIG. 3(K) by further performing waveform conversion processing such as a difference and differential operations on the waveforms of the positive and negative bar-like GCR signals. Thus, a pulse signal having a waveform extracted from a leading edge portion of the waveform of the bar signal is obtained as a reference pulse. This reference pulse fully contains energy components (i.e., signal components) of which frequencies range from 0 to 4 megahertz (MHz).
Further, FIG. 3(L) shows an example of a waveform in case where a common-mode ghost is generated. A delay time tg and an amplitude and so forth of the ghost are obtained from a sequence of error signals (.epsilon..sub.n) of FIG. 3(N) obtained by subtracting a reference waveform of FIG. 3(M) synchronized with the reference pulse from the waveform of FIG. 3(L). Thereby, tap-coefficients (i.e., gains of taps) of a transversal filter are determined and further a weight of the filter is set. This is an operating principle of a ghost cancelling device.
Referring next to FIG. 2, there is shown a typical example of a conventional ghost cancelling device according to the above described operating principle. In FIG. 2, reference numeral 8 denotes an analog-to-digital (A/D) conversion circuit; 9 a timing signal generating circuit; 11 a filtering portion which is a transversal filter composed of an FIR (Finite Impulse-Response) filter, an IIR (Infinite Impulse-Response) filter and so on; 12 a coefficient setting circuit; 13 a waveform extracting circuit; 14 an edge detecting circuit; 15 a synchronizing addition circuit; 16 a waveform converting circuit; 17 a subtracter; and 18 a reference waveform generating circuit. The timing signal generating circuit 9 generates a timing signal from horizontal synchronization and vertical synchronization signals and burst signal included in input video signal and further supplies the generated timing signal to the reference waveform generating circuit 18 to thereby synchronize the waveform extracting circuit 13 with the reference waveform generating circuit 18. Incidentally, although analog expressions are used in the following explanations for simplicity of description, devices and circuits operate with signals sampled by using a sampling frequency of which the value is equal to, for instance, 4f.sub.sc in practice. Here, f.sub.sc designates a chrominance subcarrier frequency and is nearly equal to 3.58 MHz.
In the conventional ghost cancelling device 1 of FIG. 2 first, an input video signal X(t) supplied through an input line l.sub.1 is converted by the A/D converting circuit 8. Then, the converted signal is fed to the filtering portion 11 made up of a recursive or nonrecursive transversal filter. Further, values of conversion coefficients used in the filter are controlled by the coefficient setting circuit 12. Thereafter, an output of the filtering portion 11 is supplied through a line l.sub.2 to the waveform extracting circuit 13 as an output video signal Y(t). By the waveform extracting circuit 13, a part of the output video signal including the reference signal and corresponding to a predetermined period of time (e.g., a period of time corresponding to one horizontal scanning line) is extracted. In passing, the waveform extracting circuit 13 may be constructed by using a delay circuit (not shown) for delaying the converted input video signal by a period of time corresponding to four fields and a subtracter (not shown) for effecting a subtraction between the converted input signal and the delayed signal. Thereby, the waveforms of the bar-like GCR signal as shown in FIGS. 3(I) and (J) are obtained from the reference signal as shown in FIGS. 3(A)-(H). Then, the waveform extracting circuit 13 outputs the obtained bar-like GCR signal to the edge detecting circuit 14 and the synchronizing addition circuit 15 of the next stage. The edge detecting circuit 14 detects a middle point of the leading edge portion of the bar-like GCR signal from a position at which the GCR signal has a maximum amplitude determined by effecting difference processing. Further, by utilizing the detected middle points, the edges of the bar-like GCR signals supplied to the edge detecting circuit 14 every fields are adjusted to a corresponding position in each of the fields. Incidentally, in case of the waveform as shown in FIG. 3(J), the polarity is inverted. Subsequently, the GCR signals are added up in a synchronized manner by the synchronizing addition circuit 15.
Next, the waveform converting circuit 16 generates a pulse-like waveform as shown in FIG. 3(K) by performing waveform conversion processing such as a difference and differentiation on the waveform of the bar-like GCR signal as shown in FIG. 3(I) of which the signal-to-noise ratio (S/N) has been improved by the addition effected by the synchronizing addition circuit 15 and further supplies the generated pulse to a positive input terminal of the subtracter 17. That is, the leading edge portion of the waveform of the bar-like GCR signal is used for generating a reference pulse of which the waveform reflects. transmission characteristics of the device.
On the other hand, the reference waveform generating circuit 18 generates an intrinsic reference signal (hereinafter sometimes referred to as an internal reference signal) as shown in FIG. 3(M) and supplies the generated internal reference signal to a negative input terminal of the subtracter 17. Thus, in the subtracter 17, a comparison (i.e., a subtraction) between the waveform of the pulse-like signal inputted to the positive input terminal thereof and that of the internal reference signal inputted to the negative input terminal thereof. As a result of this subtraction, the waveform including only a ghost as shown in FIG. 3(N) is obtained. The subtracter 17 outputs the result of this subtraction to the coefficient setting circuit 12 of the next stage as a sequence of error signals (.epsilon..sub.n). The coefficient setting circuit 12 has a function of determining tap-coefficients W.sub.n of the transversal filter of the filtering portion 11. For example, the coefficient setting circuit 12 determines the tap-coefficient W.sub.n by successively and repeatedly calculating the following recurrent formula by using the error signals .epsilon..sub.n : EQU W.sub.n.sup.(v+1) =W.sub.n.sup.(v) -.alpha..sup.(v) .epsilon..sub.n.sup.(v)( 1)
where .nu. denotes the number of the calculation already performed and .alpha. ordinarily designates a constant less than 1. Incidentally, .alpha. may be changed depending on .nu.. Thus, a ghost component as illustrated in FIG. 3(N) becomes smaller than a limit of visual perception. That is, the filtering portion 11 outputs video signals from which a ghost is cancelled (or reduced). In passing, the coefficient setting circuit 12 should have a function of performing an operation (i.e., a calculation) as described above and is accordingly constructed by a microcomputer, a microprocessor or the like.
In addition, the circuit of FIG. 2 is an example of a ghost concelling device of a feedback control type in which an extraction of a reference signal is performed by a part thereof at the side of an output of the filtering portion 11, and tap-coefficients W.sub.n are successively updated. However, there is another ghost cancelling device of a feedforward control type in which a extraction of a reference signal is performed by a part thereof at the side of an input of the filtering portion 11, and outputs of the filter obtained from tap-coefficients W.sub.n already calculated are not used to determine gains of taps. In each of the feedback control type and the feedforward control type of the ghost cancelling devices, a random noise component included in a part of video signals corresponding to a reference signal is reduced by the synchronizing addition circuit 15.
As above described, in the conventional ghost cancelling device 1, the waveform extracting circuit 13 performs a waveform extracting function by extracting a bar-like GCR signal as a detection reference signal from the difference between an input signal at a given moment and another signal (i.e., a four-field delay signal) preceding the input signal by a period of time corresponding to four fields (i.e., from the result of a subtraction between the input signal at a given moment and the delay signal). However, there may be a case where the period of time by which the input signal and the delay signal are separated from each other is not accurately equal to a period of time corresponding to four fields due to a flucturation in clock pulses which is caused by jitter noises included in video signals, a fluctuation in a video signal due to a change of a scene for another effected in a TV station, an impulse-like disturbance pulse noise and so on. In such a case, an unnecessary signal, which ought not to exist, may be generated under the influence of a cancelling error, which may occur due to the fact that the difference between the input video signal and the delay signal is not accurately equal to a period of time corresponding to four fields, and of a ghost if exists. This results in an erroneous detection of an unnecessary signal and in a malfunction of the device. Moreover, the conventional ghost cancelling device has a drawback that it has no countermeasures for preventing such a malfunction thereof.
The present invention is created to eliminate the above described drawback of the conventional ghost cancelling device.
It is accordingly an object of the present invention to provide a ghost cancelling device which can prevent a malfunction even when a disturbance in an input video signal is caused by jitter noises and so forth.